I have read many different thread posts giving advice on how to set the gain on a mixing console. The options presented have been interesting, to say the least, sometimes humorous and a significant amount of time just wrong. So, this is an attempt to present the concept and application of gain structure in layman’s terms to the degree that it can be done.
First, what exactly is the gain structure of a sound system? It is setting the input levels of each device in the system; mixer, processor(s) and amp(s) in such a way that the sound in the room is loud enough for the application without pushing the levels of any device in the chain into distortion or over their capability. In most applications the processor(s) and amp(s) levels will be set by the system installer and will not need attention from the sound engineer. A quick side bar on this is that if the amp(s) or speaker(s) are undersized pushing the mixer or the processor will not be able to overcome the deficiencies. I have seen sound engineers drive their mixer levels in the red or in other words, distortion, in an effort to bring the room sound up to desired levels. Obviously, this is not the right solution.
Let’s address how to set up a mixer correctly: First, we must establish that from an operational standpoint we want the faders to be able control the levels of the device on each channel from no level to the max we would ever use. That is not to say that the fader is to remain at 0db during the entire time the channel is in use. I have heard that concept given as a rule of good engineering, it is not.
Second, we must understand the function of the Gain control on a mixer channel. To fully understand this we must understand that it’s function is to match the levels of disparate input sources to each other. It is not another volume control per se. An iPad and a microphone do not have the same output levels (voltages) therefore if they are both connected and the gain is set the same on both channels the fader on the one will be higher than the other to reach an equitable level in the room.
In addition to the various device output levels, there are infinitely different levels of the program materials, i.e. not all music tracks are recorded at the same level and not all singers sing at the same level and not all persons speaking talk at the same level. And not all mics output the same level even from the same models taken out of the box at the same time. So the purpose of the Gain control on the mixer is to bring all of those variables into alignment so that the sound engineer can have proper control of the mix throughout the entire event. The question is how to do that and here are some steps that I use. Some are easier than others to administer so individual adaptations will be in order.
It is assumed for this exercise that we are addressing setting levels on microphones and not other devices that may be connected such as guitars, drums or other musical instruments. While the basic concepts remain, setting procedures will be different.
For this instruction we are addressing only one channel with a microphone connected. To revisit a goal from above, we want to end up with the fader on 0db without feedback after we have completed this routine. First, set the gain control all the way counterclockwise. Make sure that all controls in the EQ section are set to twelve o’clock. And make sure all of the monitor (Aux) levels are all the way down. Once those steps are complete, push the fader all the way to the top of its path. Slowly turn the Gain control up or clockwise until you hear the first signs of feedback and then back it down ever so slightly. Now pull the fader down to the 0 db mark on the indicator scale. For most applications this will be adequate to provide the max needed level control with the fader and you can be confident that the possibility of feedback is minimized. Also be advised that with each mic that is open there is a system gain of 3db toward feedback. If you are using multiple mics, especially in close proximity to each other you may need to perform the above routine with more than one mic open at the same time setting them each simultaneously as indicated above.
As with everything in this world there is always a variable. It may be that after you have set the mic(s) as suggested above you have a singer who is singing at 110db and uses the mic close to the mouth (that is the correct way to use a mic, by the way) and with that, the input level is way above the other singers in the group. How do you set that gain? At this point, hopefully during a rehearsal, we need to adjust the gain so that at the softest point in the music the fader is at or near 0db. This will give the ability to pull the fader down when the singer is exceeding acceptable levels for the mix but have enough headroom at hand for softer passages if needed. Again, there is nothing untoward in having a fader operating anywhere on the scale as needed. There is no need to continually reset the gain. One properly set there is no need to readdress that setting.
How about that peak or clip light next to the Gain control? If your mixer has a clip light or an LED strip indicating input levels, it is used to assist with knowing when the input levels are causing clipping in the signal chain. If the indicator shows the channel is clipping then the Gain must be reduced until the clipping stops. For typical mic usage there is no need to ask the user to pull away from the mic or sing/talk softer. Make the adjustment at the gain control. If the input that is clipping is connected to an instrument or amplifier output it is possible that too much voltage is being sent from the device to the mixer. In that case you will have clipping no matter where the Gain control is set. The resolution for this situation is to reduce the levels from the device to the mixer. There are too many variables in this scenario to address in this format.
So what about the very soft singer? Using the first method above is really all you can do because any additional gain will introduce feedback. Obviously getting the singer closer to the mic will make the program material more available to the mic to be picked up but if that can’t be accomplished there is nothing else that can be done. Maybe I’ll address mic usage in a future post. Feedback control might be another topic to address that fits in with this one.